ICOS is a member of the B7/CD28/CTLA-4 immunoglobulin superfamily and is specifically expressed on T cells. Unlike CD28, which is constitutively expressed on T cells and provides co-stimulatory signals necessary for full activation of resting T cells, ICOS is expressed only after initial T cell activation.
ICOS has been implicated in diverse aspects of T cell responses (reviewed in Simpson et al., 2010, Curr. Opin. Immunol., 22: 326-332). It plays a role in the formation of germinal centers, T/B cell collaboration, and immunoglobulin class switching. ICOS-deficient mice show impaired germinal center formation and have decreased production of interleukin IL-10. These defects have been specifically linked to deficiencies in T follicular helper cells.
ICOS also plays a role in the development and function of other T cell subsets, including Th1, Th2, and Th17. Notably, ICOS co-stimulates T cell proliferation and cytokine secretion associated with both Th1 and Th2 cells. Accordingly, ICOS KO mice demonstrate impaired development of autoimmune phenotypes in a variety of disease models, including diabetes (Th1), airway inflammation (Th2) and EAE neuro-inflammatory models (Th17).
In addition to its role in modulating T effector (Teff) cell function, ICOS also modulates T regulatory cells (Tregs). ICOS is expressed at high levels on Tregs, and has been implicated in Treg homeostasis and function.
Upon activation, ICOS, a disulfide-linked homodimer, induces a signal through the PI3K and AKT pathways. Subsequent signaling events result in expression of lineage specific transcription factors (e.g., T-bet, GATA-3) and, in turn, effects on T cell proliferation and survival.
ICOS ligand (ICOSL; B7-H2; B7RP1; CD275; GL50), also a member of the B7 superfamily, is the only ligand for ICOS and is expressed on the cell surface of B cells, macrophages and dendritic cells. ICOSL functions as a non-covalently linked homodimer on the cell surface in its interaction with ICOS. Human ICOSL, although not mouse ICOSL, has been reported to bind to human CD28 and CTLA-4 (Yao et al., 2011, Immunity, 34: 729-740).